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Spinal Cord Motor Control

Jul 10, 2025

Overview

This lecture reviews chapter 55 of Guyton and Hall’s Medical Physiology, focusing on the spinal cord’s role in controlling motor function and reflexes.

Organization of Motor Control

  • Simple muscle reflexes originate in the spinal cord; complex movements involve the brainstem and cerebrum.
  • The sensory (dorsal) root brings signals into the spinal cord, connecting to interneurons and anterior (motor) neurons for reflexes.
  • Motor responses may remain at spinal level or ascend to higher brain centers for advanced control.

Types of Motor Neurons

  • Alpha motor neurons innervate extrafusal (skeletal) muscle fibers for overall muscle contraction.
  • Gamma motor neurons innervate intrafusal muscle fibers (within muscle spindles) to regulate muscle tone.
  • Renshaw cells provide lateral inhibition to fine-tune motor signals.

Muscle Spindles and Stretch Reflex

  • Muscle spindles detect changes in muscle length and the speed of that change.
  • Two types of intrafusal fibers: nuclear bag fibers and nuclear chain fibers.
  • Primary afferents (Ia) detect both length and rate of change; secondary afferents (II) detect only length.
  • Gamma d (dynamic) motor neurons adjust for rapid changes; gamma s (static) for sustained changes in length.
  • Stretch reflex is monosynaptic: muscle stretching excites spindle, leading to contraction (e.g., patellar reflex).
  • Co-activation of alpha and gamma motor neurons ensures smooth muscle contraction and posture control.

Golgi Tendon Organs and Inverse Stretch Reflex

  • Golgi tendon organs, located in tendons, sense muscle tension and rate of change in tension.
  • High tension activates inhibitory signals to relax the muscle, preventing damage (inverse stretch reflex).
  • Both muscle spindles and Golgi organs send information to higher centers for proprioception.

Spinal Cord Reflexes

  • Flexor (withdrawal) reflex removes limb from painful stimulus via excitatory interneurons and reciprocal inhibition.
  • Crossed extensor reflex triggers extension in the opposite limb to maintain balance during withdrawal.
  • Reflex arcs can be complex, coordinating multiple muscles for proper retraction or movement.

Postural and Locomotive Reflexes

  • Pressure on limbs triggers reflex extensions (magnet reactions) for maintaining posture, even in spinal cord injuries.
  • Stepping and walking can occur via local spinal circuits without input from the brain.
  • The scratch reflex, muscle spasms, and cramps result from local spinal cord reflex arcs.

Autonomic and Mass Reflexes

  • Visceral reflexes (e.g., bladder emptying, sweating) are controlled via spinal reflexes.
  • Mass reflex occurs after spinal cord injury, causing widespread motor and autonomic activity due to loss of inhibitory control.

Spinal Shock

  • Severance of the spinal cord causes temporary loss of all reflexes below the injury, with gradual recovery over time.

Key Terms & Definitions

  • Alpha motor neuron — neuron innervating skeletal (extrafusal) muscle fibers for contraction.
  • Gamma motor neuron — neuron innervating intrafusal muscle fibers inside muscle spindles.
  • Muscle spindle — sensory receptor detecting muscle length and speed of stretch.
  • Golgi tendon organ — receptor in tendons sensing muscle tension.
  • Stretch reflex — monosynaptic reflex causing muscle contraction in response to stretch.
  • Reciprocal inhibition — process where antagonist muscles are inhibited during reflex contraction.
  • Proprioception — awareness of body position and movement.
  • Spinal shock — temporary loss of reflexes after spinal cord injury.

Action Items / Next Steps

  • Review diagrams illustrating spinal cord reflex arcs and receptor locations.
  • Read the next chapter on higher motor function control pathways.

Certainly! Here's a highly detailed and expanded bullet-point summary based closely on the video content, with added depth and clarity:

Detailed Summary of Chapter 55: Spinal Cord Control of Motor Function

  • Introduction to Motor Control Hierarchy

    • Motor responses begin in the spinal cord for simple muscle reflexes.
    • More complex motor responses involve the brainstem.
    • The most complicated muscle skills are controlled by the cerebrum.

  • Spinal Cord Organization for Motor Function

    • Sensory (dorsal) root is located on the top of the spinal cord.
    • Sensory nerves enter through the dorsal root and synapse in the gray matter.
    • Interneurons in the gray matter connect sensory inputs to anterior motor neurons.
    • Local reflexes involve direct connections from sensory neurons to motor neurons.
    • Some sensory signals ascend to higher brain centers for complex motor control.

  • Types of Motor Neurons

    • Alpha motor neurons: Innervate extrafusal skeletal muscle fibers, controlling large muscle groups.
    • Gamma motor neurons: Innervate intrafusal muscle fibers within muscle spindles, regulating muscle tone.
    • Renshaw cells: Inhibitory interneurons providing lateral inhibition to fine-tune motor output and prevent signal spread.

  • Muscle Spindles and Their Function

    • Muscle spindles are specialized sensory receptors within muscles that detect changes in muscle length and the rate of lengthening.
    • Intrafusal fibers inside muscle spindles include:
      • Nuclear bag fibers: Clustered fibers sensitive to dynamic changes.
      • Nuclear chain fibers: More evenly distributed fibers sensitive to static length.
    • Sensory afferents:
      • Primary afferents (Ia fibers): Innervate both nuclear bag and chain fibers; detect both length and rate of lengthening (dynamic and static responses).
      • Secondary afferents (II fibers): Innervate nuclear chain fibers only; detect static length changes.
    • Gamma motor neurons have two types:
      • Gamma d (dynamic): Control dynamic response, adjusting spindle sensitivity to rapid length changes.
      • Gamma s (static): Control static response, maintaining spindle sensitivity during sustained stretch.
    • Stretching the muscle spindle excites sensory neurons, sending signals to the spinal cord.
    • The stretch reflex is a monosynaptic reflex arc:
      • Sensory neuron excites alpha motor neuron.
      • Alpha motor neuron causes contraction of the stretched muscle.
    • Example: Patellar reflex (knee-jerk) demonstrates this mechanism.
    • Co-activation of alpha and gamma motor neurons ensures smooth, controlled muscle contraction and posture maintenance.
    • Signals to gamma motor neurons originate from brainstem regions (bulbo reticular facilitory area), cerebellum, basal ganglia, and cerebral cortex.

  • Golgi Tendon Organs and Their Role

    • Located in tendons, Golgi tendon organs sense muscle tension and rate of tension change.
    • When tension is excessive, Golgi tendon organs send inhibitory signals to relax the muscle, preventing damage.
    • This inverse stretch reflex helps distribute load across multiple muscles.
    • Both muscle spindles and Golgi tendon organs send proprioceptive information to higher brain centers via the spinal cerebellar tract.

  • Spinal Cord Reflexes

    • Flexor (withdrawal) reflex:
      • Triggered by painful stimuli.
      • Excitatory interneurons activate flexor muscles to withdraw the limb.
      • Reciprocal inhibition suppresses antagonist muscles to allow smooth withdrawal.
      • After-discharge in interneurons prolongs the withdrawal response for safety.
    • Crossed extensor reflex:
      • Contralateral limb extends to maintain balance during withdrawal.
      • Complex interneuronal circuits coordinate multiple muscles for appropriate movement direction.

  • Postural and Locomotive Reflexes

    • Pressure on footpads triggers reflexive limb extension to maintain posture.
    • Magnet reaction: Increased extension on the side receiving pressure.
    • Even with spinal cord transection, animals can perform stepping and walking motions via spinal reflex circuits.
    • Stumble reflex: Lifting the foot when the top is touched during walking.
    • Reciprocal stepping involves alternating limb movements coordinated at the spinal level.
    • Scratch reflex allows animals to locate and scratch irritants using spinal reflexes.

  • Muscle Spasms and Cramps

    • Local irritation or metabolic abnormalities cause reflexive muscle contractions.
    • Positive feedback from sensory receptors intensifies spasms or cramps.
    • Similar reflex mechanisms occur in the autonomic nervous system (e.g., vascular tone changes, sweating, bladder emptying).

  • Mass Reflex and Spinal Shock

    • Mass reflex: After spinal cord injury, loss of inhibitory neurons causes widespread motor and autonomic discharge (muscle spasms, hypertension, sweating).
    • Spinal shock: Temporary loss of all reflexes below the injury site, lasting hours to weeks before reflexes gradually return.

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